1. Field of the Invention
This invention relates to a method for enhancing the bondability of a metallized substrate having conductive material electroplated thereon. In particular, the invention is directed to treating the metallized substrate, prior to electroplating the conductive material thereon, in order to enhance the bondability of the resulting conductively coated metallized substrate.
2. Description of the Prior Art
Thin film electrical circuits may be made by evaporating or sputtering very thin metal films onto an insulative substrate, then a coating of electrically conductive material is evaporated onto the thin films to improve the conductivity of the metallized substrate. Portions of the conductively coated metal films are then removed to form the desired thin film electrical circuit.
Fabrication of such thin film electrical circuits is initiated by sputtering a metallic base layer on the surface of the insulative substrate. Portions of the base layer will be processed subsequently to form thin film components, such as resistors and capacitors. An electrically conductive layer of material on the base layer is required to provide reliable conductive connections between such components and to protect the underlying thin metal films. However, such conductive layer material does not adhere well to the base layer. In order to provide acceptable adhesion, a glue layer of metal is deposited onto the base layer and a metallic diffusion barrier layer is deposited onto the glue layer to prevent undesirable diffusion of the glue layer into the conductive layer.
Normally, the electrically conductive layer is sputtered or evaporated onto the diffusion barrier layer by well-known techniques. Although evaporation and sputtering provide acceptable bondability of the resulting thin film substrates, these deposition methods are slow, inefficient and require complicated and expensive equipment. Such methods result in only a small portion of the material which is used to form the conductive layer being deposited on the metallized substrate for the direction of sputtered or evaporated material is subject to limited control. Although such techniques may be tolerated when evaporating or sputtering material other than precious metals, it is desirable to use more efficient methods when depositing materials such as gold or the like. One such method is electroplating which, advantageously, limits deposition of material to the specific object to be plated.
In recent years attempts have been made to selectively electroplate gold onto thin film metallized substrates in order to conserve this precious metal. This is accomplished by covering the metallized substrate with a liquid photoresist, applying a circuit pattern mask to the surface of the photoresist, exposing the unmasked portions to ultraviolet light and developing away selected portions of the photoresist to form a circuit pattern therein. Gold is then electroplated onto the metallized portions of the substrate from which the photoresist has been selectively removed. The photoresist is then stripped off and the electroplated gold is used as an etch mask as the thin metal films, not protected by the gold, are removed with etchants. Thus, the end product of such a selective plating technique is a gold coated thin film circuit on an insulative substrate. Although the bondability of the resulting thin film circuit was acceptable, it has been found that in such a selective electroplating process there is a breakdown of the photoresist along the edges which define the areas to be gold plated. The electroplated gold creeps under the photoresist, along the surface of the underlying thin film, resulting in poor definition and unreliable thin film circuitry.
An additional method of forming thin film circuits is to electroplate gold on the entire surface of the metallized substrate. Once coated with gold, the surface of the gold is coated with photoresist. A mask is then placed over the photoresist and selected portions thereof exposed and developed away. The undeveloped photoresist protects the desired circuit pattern as the exposed gold and the metallization thereunder is etched away. the undeveloped photoresist is then stripped off leaving only the gold plated thin film circuit pattern on the insulative substrate.
Although this method advantageously precludes the creeping of electroplated gold onto the underlying thin films and results in acceptable circuit definition, an additional problem arose. It was found that the adhesion at the interface of the aforementioned glue layer and the diffusion barrier layer deteriorated causing unacceptably low bond strengths of leads which are bonded to the resulting gold plated thin film circuit conductors and subjected to a90.degree.pull test.